High voltage and high power, 50 Hz or 60 Hz, 3-phase, long-distance electrical power transmission lines are common. They typically consist of 3 or 6 groups of wires with one or two groups of wires for each of the 3 phases. Each group of wires can contain 1-4 wires for each of the 3 phases. The groups of wires are usually suspended from very tall metal towers by long insulators. These transmission lines operate at high voltages, such as 115 kV, 230 kV and 765 kV and at power levels up to 1-2 Giga watts.
Conventional electrical power transmission lines operating above 230 kV usually consist of three groups of wires on one side of the metal tower and another three groups of wires on the other side. Due to the inherent asymmetry of the arrangement of the groups of wires, some phases experience an electrical transmission impedance that is higher than the impedance for other phases. Often this impedance imbalance is corrected along the length of the transmission line by adding capacitance to null the unequal amounts of inductance created by the transmission line for each phase.
Not only is the impedance of conventional electrical transmission lines unbalanced, it is also high compared to the resistance of the load. Therefore, the transmission line is highly inductive. This high inductance must be balanced by adding capacitors along the length of the transmission line for an efficient transmission of power.
Conventional transmission lines mounted on lattice towers require a large right of way, are unsightly, and create significant magnetic fields and health risks in their local area. They also are expensive to maintain and subject to outages due to high winds, ice storms and snow. They are relatively expensive—about $1.6 M per GW per km of length for long distance transmission lines. The percentage of the cost of a conventional transmission line that is devoted to the active elements (the wires) is only about 12%. Thus, a radically new design could dramatically lower the cost of transmitting electrical power over long distances.
Underground transmission lines also exist, but are more expensive than conventional above ground transmission lines. For high voltages and high power, it is expensive to insulate the underground conductors, to shield them from water penetration and to provide for sufficient thermal conductivity to enable the conductors to carry high power.
In the prior art, extensive work has been done with gas insulated three-phase transmission lines for high voltage, high power and long-distance electricity transmission. Most of this prior art uses conductors that are large, hollow, cylindrical tubes, rather than stranded wire. The tubes are in multiples of three, arranged in a triangular cross-section, and located inside a cylindrical metal conduit. The combination of the symmetric, triangular arrangement of conductors and the identical geometry from each conductor to the enclosing metal conduit should assure that the impedance of each conductor is the same.
Unfortunately, practical implementations of large conductors, positioned symmetrically inside a cylindrical metal tube and insulated for high voltage are difficult to achieve and expensive. To transmit power levels above 10 GW, the conducting tubes need to be large. The conducting cylindrical conduit that encloses the tubes needs to be even larger. To preserve the symmetry, the insulators need to have triangular, hexagonal or higher order symmetry. Thus, the entire structure becomes very difficult to implement over a long distance at a competitive cost.
Rectangular geometries are much more practical to implement at a reasonable cost. However, while obtaining an impedance balance for a 3-phase electrical transmission system is conceptually easy with multiples of 3 conductors arranged in a cylindrically symmetrical geometry, it is very difficult in a rectangular geometry. An array of conductors in a rectangular geometry will have one conductor at each end. The impedance of these end conductors will be different than the impedance of any of the conductors in the middle of the array. Since there are only two ends, it is not possible to have one of the end conductors carry each of the 3 phases. Hence, the conductors for each phase will not have the same impedance and there will be an imbalance among the phases. Also, the electric current tends to flow on conductor surfaces that are adjacent to other conductor surfaces. Thus, the outsides of the end conductors will carry little current, which is inefficient and costly.
In the prior art, there are examples of large, hollow, rectangular bus bars used as conductors for transmitting 3-phase electrical power over short distances and at low voltages to electric arc furnaces. Electric arc furnaces depend on a careful balance among the impedance of the transmission system for each phase. Various compensation schemes have been developed for creating a balance among the 3-phases, even though the transmission system itself is not balanced among the 3 phases. This compensation approach may be acceptable for the short, low-voltage, high current transmission system required for an electric arc furnace. However, it is not practical or cost effective for a long-distance, high current and high voltage electrical transmission line.
There are other issues that must be solved for long-distance, high voltage and high power electrical transmission lines. Conventional above-ground transmission lines have a very large air space between conductors to minimize electric arcs triggered by fog, falling rain and snow. Conventional underground transmission lines are wrapped with many layers of solid insulation (e.g. plastic, tar paper, etc). Underground lines that are wrapped with insulation have great difficulty implementing an insulation that can withstand extremely high voltage (EHV or 765 kV), that has good thermal conductivity to carry away heat, and that provides sufficient protection from water incursion. Also, this insulation wrapping is expensive.
Finally, a high power electrical transmission structure would require expansion joints for thermal expansion that must flex at least 36,500 times over 100 years. These joints must also conduct very high currents (circa 10,000 amps) and have smooth surfaces that will minimize corona discharge. Prior art solutions are based on flexible, high current conductors like woven cables and bundles of thin sheets of copper that offer flexibility. However, they are often too expensive, will not flex 36,500 times with a very low probability of breaking, and can have edges that promote corona discharge. There are also prior art solutions for thermal expansion of oil and gas pipelines, as shown in FIG. 5. Unfortunately, the solutions used for oil and gas pipelines cannot be used for electrical transmission lines, when the spacing between adjacent lines is small and must be precisely controlled.